1. Field of the Invention
The present invention relates to an after-stop fuel pressure control device of a direct injection engine having a pressure reducing mechanism for reducing pressure of fuel (fuel pressure) in a high-pressure fuel system after an engine stops.
2. Description of Related Art
A time from injection to combustion of a direct injection engine that injects fuel directly into a cylinder is shorter than that of an intake port injection engine that injects fuel to an intake port. Accordingly, the direct injection engine cannot have a sufficient time for atomizing the injected fuel and is required to increase injection pressure to high pressure to atomize the injected fuel. A certain direct injection engine (for example, refer to Patent document 1. JP-A-2005-264902) is structured such that fuel drawn from a fuel tank with a low-pressure pump is supplied to a high-pressure pump driven by a camshaft of the engine and the high-pressure fuel discharged by the high-pressure pump is pumped to an injector through a high-pressure fuel pipe.
Generally, the high-pressure pump has a check valve for preventing a backflow of the discharged fuel, thereby maintaining the fuel pressure in the high-pressure fuel pipe at high pressure. However, if the fuel pressure in the high-pressure fuel pipe is maintained at the high pressure after the engine stops, a quantity of fuel leakage from the injector (i.e., oil-tightness leak amount) tends to increase during the engine stoppage. As a result, there occurs a problem that the leak fuel accumulates in the cylinder and is discharged in an unburned state in next starting, deteriorating an exhaust emission at the starting.
As measures against such the problem, the engine described in Patent document 1 is provided with an electromagnetic relief valve in a predetermined position of a high-pressure fuel system (such as a delivery pipe, the high-pressure fuel pipe and the high-pressure pump) that supplies the high-pressure fuel from the high-pressure pump to the injector. After the engine stops, the relief valve is opened to reduce the fuel pressure in the high-pressure fuel system to pressure on the low-pressure pump side.
A fuel pressure control device of a direct injection engine described in Patent document 2 (JP-A-2005-207339) controls a fuel pressure control valve immediately before stopping an engine such that the engine is stopped after fuel pressure in a high-pressure fuel system is reduced to after-stop target fuel pressure set in consideration of startability and the like. No electromagnetic relief valve is provided in this system.
In recent years, in order to shorten a starting time, it has been a technical problem of the direct injection engine to realize single compression starting of starting the engine by causing the first explosion in a cylinder, which is in the first compression stroke in the starting. However, there is a possibility that the technology described in Patent document 1 or 2 fails in the single compression starting because of following reasons depending on an engine stop position.
During the engine stoppage, a compressed air in the cylinder in the compression stroke leaks from gaps of a combustion chamber (such as a gap around a piston and a gap around a suction valve or an exhaust valve) and cylinder pressure decreases. If cranking is started thereafter, the air in the cylinder in the first compression stroke is compressed again when a piston ascends. The initial position of the piston as of the start of the cranking varies with engine stop timing. Therefore, the maximum cylinder pressure in the cylinder in the first compression stroke (i.e., the cylinder pressure at the time when the piston of the cylinder in the first compression stroke ascends to the top dead center) varies with the initial position of the position as of the start of the cranking.
The technology of Patent document 2 uniformly sets the after-stop target fuel pressure regardless of the engine stop position. Therefore, depending on the initial position of the piston as of the start of the cranking, there is a possibility that the initial fuel pressure as of the start of cranking (i.e., the pressure for injecting the fuel into the cylinder in the compression stroke) does not become the optimum fuel pressure for the single compression starting. Therefore, for example, if the initial fuel pressure as of the start of the cranking becomes excessively low, there is a possibility that the fuel injection pressure becomes insufficient with respect to the maximum cylinder pressure of the cylinder in the first compression stroke and the single compression starting fails. If the after-stop target fuel pressure is set relatively high as measures against such the problem, there can occur a problem that the fuel leakage (oil-tightness leakage) from the injector during the engine stoppage increases.